As is known in the art, DC-DC converters are used in a wide range of applications, such as to provide a regulated output voltage from a battery or other power source. Switch-mode or switching DC-DC converters use an energy storage device, such as an inductor, to store the input energy, and one or more switches to selectively couple the energy storage device to the output.
Systems may include a power bus to supply various circuit boards and circuits with power. For example, vehicles may have a power bus that is impacted by cold cranking of an engine, change-overs between gasoline power and battery power, load dump conditions, and the like. Such events may cause relatively high voltage supply signal transients during vehicle operation. For example, automobiles can have a power bus that experiences 16V to 8V transients at 1 V/μs rate. These types of input transients may present challenges for providing DC-DC converters that meet certain line transient and regulation requirements.
FIG. 1 illustrates a conventional voltage mode Buck-Boost converter 10 in which an inductor 14 is selectively coupled to an input voltage source 18 to store energy when switches Q1 and Q2 are both on or decoupled from the input voltage source to transfer energy to the load Ro at the converter output 22 through diodes D1 and D2 when switches Q1 and Q2 are both off in a Buck-Boost mode of operation. The converter 10 can be configured to operate in a purely Buck mode of operation by keeping switch Q2 off, as may be desirable when the input voltage is significantly higher than the desired output voltage or in a purely Boost mode of operation by keeping switch Q1 on, as may be desirable when the input voltage is significantly lower than the desired output voltage.
A control circuit 20 controls the duty cycle of conduction of the switches Q1 and Q2 based on feedback from the converter output 22 in order to provide the output voltage Vout at a desired, predetermined regulated voltage level. For example, the control circuit 20 may include an error amplifier 24 to generate an error signal Vc, 26 based on a difference between the converter output 22 and a reference signal Vref. The control circuit 20 may further include a comparator 28 to compare the error signal Vc to a Pulse Width Modulation (PWM) ramp signal to generate a switch control signal 30 for coupling to the gate terminal of switches Q1 and Q2. In some converters, the switches Q1 and Q2 may be independently controlled, in which case an additional comparator 34 may be provided to generate a control signal 36 to the gate terminal of switch Q2. Such independent control of the switches Q1 and Q2 permits the converter 10 to be configured in a Buck mode of operation or in a Boost mode of operation and also permits a third phase of Buck-Boost operation (i.e., operation with one of the switches Q1 and Q2 on and the other one off) as may be desirable to reduce losses and improve efficiency when transitioning between switch phases.
FIG. 2 shows a conventional voltage mode Boost converter 50 having some commonality with the prior art converter of FIG. 1. An impedance IMP provided by one or more resistive and capacitive components, for example, controls characteristics of an output voltage VOUT feedback signal. Slope compensation is provided for the ramp signal to a PWM comparator that controls a duty cycle of the switching element in the boost stage.
For the prior art boost converter of FIG. 2, FIG. 2A shows a waveform for Vin, FIG. 2B shows a waveform for COMP (I/O pin), FIG. 2C shows a waveform for VOUT, and FIG. 2D shows a waveform for IL (inductor L current). FIG. 2A shows a transient on Vin that drops the voltage level from 16V to 8V for about time=2 sec to about time−3.5 sec. FIG. 2B shows in response to the Vin voltage transient an overshoot and then undershoot on the COMP signal. FIG. 2C shows the output voltage VOUT response which ramps up rapidly to about 55V from about 30V at about time=3.5 sec and decays thereafter until reaching about 30V. FIG. 2D shows the inductor current IL which spikes to about 18V as the VOUT signal ramps up. Such signal behavior may not meet certain input voltage transient response requirements.
While conventional converters may be suitable for some applications, it may provide less than optimal performance for a regulated output voltage in the presence of significant input voltage signal transients since the transient response may be relatively slow as the control loop relies upon an output voltage error to adjust operation of the boost circuit.